If in the divorce agreement child care costs are split 50/50, how is cost split if one parent qualifies for assistance and the other does not?

If you are under a court order to pay for 50% of your children's child care costs and your circumstances have changed since the divorce that you are now receiving some type of assistance and can no longer afford to pay for those expenses, you would be obligated to file a motion with the court to ask to be relieved of the divorce obligations. It is difficult to answer your question in any more specificity without knowing more of the facts from you.